A plasma facing component provided in a nuclear fusion reactor refers in general to equipment directly opposed to plasma, includes a divertor, a blanket surface, a limiter or the like, and receives severe heat and particle load from high temperature plasma. The first wall is required to maintain structural integrity and serve as a shield against plasma for the surrounding structure without causing negative effects to the plasma.
Therefore, the plasma facing component is required to have a function to well endure such a high heat load and remove the heat. In order to achieve the function of the first wall required for removing the heat of the high heat load, it is necessary to prepare the heat receiving equipment of the plasma facing component by using a material having an excellent heat conductivity.
FIG. 8 is a sectional view illustrating one example of heat receiving tile which is used in a divertor of a nuclear fusion reactor.
Especially the divertor of the Tokamak type nuclear fusion reactor receives the highest heat load among the equipment provided in a nuclear fusion reactor because the kinetic energy of the charged particles coming into the divertor is applied thereto as heat. Therefore, the divertor is required to have a function for well enduring such a high heat load and removing the heat.
For protecting a cooling structure from a sputtering due to ion radiation or a heat impact caused by plasma disruption, the divertor is provided with a heat receiving block formed of a material, which has less negative effect against the plasma, on its surface facing the plasma.
The heat receiving block is preferably formed of a certain material having a low atomic number which as less negative effect against the plasma, especially a carbon fiber reinforced carbon composite material (CFC material) which is a carbon-based material having higher heat conductivity. This is because particles are generated from the surface of the heat receiving block and scattered into the plasma by the effect of the sputtering or the like, leading to temperature drop of the plasma and degradation of confinement properties of the plasma.
Further, this heat receiving block has a cooling tube provided therein to achieve the function of the divertor required for removing the heat of the high heat load.
In the nuclear fusion reactor, which is designed for performing a long-term operation, the surface temperature of a component constituting the divertor tends to exceed the melting point thereof due to the thermal flow going beyond the limits of the heat capacity of the materials. Therefore, a forced cooling approach has been employed for cooling the heat receiving block with the cooling tube formed of a copper alloy, such as chromium-zirconium copper (CuCrZr) or the like, exhibiting high heat conductivity and strength, and the heat received by the heat receiving block is forcibly removed by a coolant such as water or the like flowed through the cooling tube.
However, the heat receiving block formed of a carbon material such as CFC or the like has a poor ability of joining to the cooling tube formed of a CuCrZr copper alloy, and there is a great difference in the thermal expansion coefficient between the heat receiving block and the cooling tube.
Therefore, for conducting the heat energy received from the plasma to the cooling tube and for absorbing the difference in the thermal expansion coefficient efficiently, it is preferred to reduce the thermal resistance as much as possible by providing a material of interlayer formed of a copper material such as CuW or the like between the heat receiving block and the cooling tube and by metallurgically joining these materials such as by brazing or the like, using a joining material mainly containing Cu—Mg and Ti—Cu based material having an excellent heat conductivity.
However, the thermal expansion coefficients greatly differ from one another as follows: 2×10−6 for the heat receiving block; 2×10−5 for the cooling tube; and 1×10−6 for the material of interlayer. Therefore, defects tend to occur at joining parts during the high temperature treatment in the brazing process, especially when the cooling tube is shrinking due to temperature drop, and the material of interlayer and the heat receiving block arranged outside the cooling tube cannot follow the shrinkage of the cooling tube arranged inside. For this reason, the heat receiving block tends to be cracked, or peeling between the heat receiving block and the material of interlayer tends to occur, thereby leading to reduction in heat transfer coefficient and cooling efficiency.
Patent Document 1 discloses a high heat resistant structural component in which a graphite part and a metallic part are bonded to each other via a brazing layer, and an intermediate layer is provided between the metallic part and the brazing layer. The provision of this special intermediate layer is intended to absorb the difference in the thermal expansion coefficient between dissimilar materials, thereby firmly bonding the graphite and the metal with each other.
It is true that the high heat resistant structural component disclosed in the Patent Document 1 can endure a heat cycle load that the component will undergo during the operation of the nuclear fusion reactor. Thus, occurrence of undue deformation or cracks in this component can be prevented. However, due to the brazing at temperature between 850 and 1900° C., a high temperature process required for producing such a component tends to damage the component, thus degrading the production yield.
Various problems were found out from our study on test samples each including a cooling tube formed of a copper alloy, a heat receiving block formed of CFC material, and a cylindrical material of interlayer formed of oxygen free copper and inserted between the cooling tube and the heat receiving block. For instance, from an aging treatment at 480° C. on the test sample after it was subjected to vacuum brazing at 985° C. and then rapidly quenched to maintain the strength of the precipitation-hardening type copper alloy, we found at a considerably high frequency that the part of the heat receiving block in contact with the material of interlayer has been cracked in the axial direction, as well as found defects caused by the failure in brazing.
As such, the conventional heat receiving tile requires further improvement in the cooling function.